1. Field of the Invention
The present invention relates to a motor drive apparatus, and more particularly to a motor drive apparatus which detects the relative position between motor windings and a rotor and continuously controls the rotation of a motor.
2. Description of the Background Art
Motor drive apparatuses obtain a position signal which indicates the relative position between motor windings and a rotor, obtain a rotation signal which indicates the magnitude relation between the position signal and the reference value of the position signal, and continuously control the power to be supplied to the motor based on the rotation signal. Here, the noise generated in the motor drive apparatus may interfere the position signal and chattering may occur in the rotation signal. In order to prevent chattering which occurs in the rotation signal, conventional motor drive apparatuses employ a method of eliminating noise which is contained in the position signal, using a low-pass filter or a method of making a comparison between the position signal and the reference value using a comparator having hysteresis.
FIG. 9 is a diagram showing the configuration of a conventional motor drive apparatus. A motor drive apparatus 9 shown in FIG. 9 includes a position detection section 10; a crossing detection section 29, an energization control signal generation section 30; a pulse width modulation (hereinafter referred to as “PWM”) signal generation section 40; a gate circuit 50; and power transistors Q1 to Q6. The motor drive apparatus 9 employs a method of making a comparison between the position signal and the reference value using the crossing detection section 29 having hysteresis.
In FIG. 9, the position detection section 10 detects the relative position between motor windings L1 to L3 and a rotor 60 and outputs a position signal 101 which indicates the result of the detection. The position signal 101 and a reference signal 102 which indicates the reference level of the position signal 101 are inputted to the crossing detection section 29. The crossing detection section 29 makes a comparison between the position signal 101 and the reference signal 102 using a comparator 21 and outputs a rotation signal 291 which indicates the magnitude relation between the position signal 101 and the reference signal 102. The crossing detection section 29 changes the level of the reference signal 102 by a predetermined amount and in a predetermined direction when the magnitude relation between the position signal 101 and the reference signal 102 is reversed. Specifically, when the position signal 101 is greater than the reference signal 102, the level of the reference signal 102 is reduced by a predetermined amount, and when the position signal 101 is smaller than the reference signal 102, the level of the reference signal 102 is increased by a predetermined amount.
The energization control signal generation section 30 outputs, based on the rotation signal 291, energization control signals 301 for sequentially energizing the motor windings L1 to L3. The PWM signal generation section 40 generates a PWM signal 401 having a predetermined time width. The gate circuit 50 computes the logical AND of the energization control signals 301 and the PWM signal 401. The power transistors Q1 to Q6 supply power to the motor windings L1 to L3 based on the energization control signals 301 and the output signals of the gate circuit 50.
FIG. 10 is a signal waveform diagram showing the input and output signals of the crossing detection section 29. As shown in FIG. 10, the position signal 101 is ideally in the form of a sine wave (indicated by a dashed line in FIG. 10) whose amplitude changes periodically in accordance with the rotation of the rotor 60. The reference signal 102 is described assuming that the reference signal 102 is a constant level signal. However, in the actual position signal 101 (indicated by a solid line in FIG. 10), noise having a constant amplitude and period is included.
Therefore, in the case where the crossing detection section 29 does not have hysteresis, chattering occurs in the rotation signal 291 during a period of time before and after the magnitude relation between the position signal 101 and the reference signal 102 is reversed. On the other hand, in the case where the crossing detection section 29 has hysteresis, chattering does not occur in the rotation signal 291 even during the aforementioned period of time. Thus, by using the crossing detection section 29 having hysteresis, it is possible to prevent chattering which occurs in the rotation signal 291.
Further, as another conventional art related to the present invention, Japanese Laid-Open Patent Publication No. 2002-10678 discloses a technique of stably driving a sensorless spindle motor by setting the capacity of a capacitor which is externally mounted on a mask signal generation circuit, to a value suitable for low-speed rotation and high-speed rotation according to a control signal from a controller.
The above-described conventional motor drive apparatuses, however, have the following problems. In the motor drive apparatus which eliminates noise contained in the position signal using a low-pass filer, if the capacity of the capacitor included in the low-pass filter is increased to improve the noise elimination level, the delay time in the low-pass filter increases. This causes a delay in the output of the rotation signal, and as a result, the response to the detected position signal is reduced. In particular, in a high-speed rotation motor, a slight delay time may cause a large phase delay and thus a reduction in response leads to a big problem.
In addition, in the motor drive apparatus which makes a comparison between the position signal and the reference value using a comparator having hysteresis, the rotation signal changes after the lapse of a predetermined delay time (time T1 shown in FIG. 10) from when the magnitude relation between the position signal and the reference signal is reversed. The delay time is determined depending on the hysteresis width of the comparator but not on the number of rotations of the motor. Therefore, in the case where the position signal changes slowly, the delay time becomes longer, resulting in a delay in the output of the rotation signal. In addition, in the case where there are variations in the characteristics (e.g., a hysteresis width) of the comparator, variations also occur in a rotation signal to be outputted from the comparator. Further, in the case where the hysteresis width is smaller than the level of noise, chattering occurs in the rotation signal, and thus the level of noise needs to be pre-estimated when determining the hysteresis width of the comparator.